Update design doc and site docs to reflect ItemBlock implementation

[sseago]

Update design doc and site docs to reflect ItemBlock implementation

Note that for the existing velero plugin types, the site docs do not directly include field-by-field specifications. Instead, the user is linked to the velero-plugin-example repo for examples of each plugin type. Consistent with that, vmware-tanzu/velero-plugin-example#74 should be considered a companion PR to this one, which makes some minor updates to site docs and modifies the feature design doc to reflect changes made during phase 1 implementation.

Please add a summary of your change

Note that for the existing velero plugin types, the site docs do not directly include field-by-field specifications. Instead, the user is linked to the velero-plugin-example repo for examples of each plugin type. Consistent with that, vmware-tanzu/velero-plugin-example#74 should be considered a companion PR to this one, which makes some minor updates to site docs and modifies the feature design doc to reflect changes made during phase 1 implementation.

Does your change fix a particular issue?

Fixes #7900

Please indicate you've done the following:

• [ x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [ x] Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• [x ] Updated the corresponding documentation in site/content/docs/main.

[sseago]

/kind changelog-not-required

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.15%. Comparing base (3f9c2dc) to head (e6fb4ba).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8218   +/-   ##
=======================================
  Coverage   59.15%   59.15%           
=======================================
  Files         367      367           
  Lines       30777    30777           
=======================================
  Hits        18206    18206           
  Misses      11113    11113           
  Partials     1458     1458           

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